// P0784R7
// { dg-do compile { target c++20 } }

namespace std
{
  inline namespace _8 { }
  namespace _8 {

    typedef __SIZE_TYPE__ size_t;

    template <typename T>
    struct allocator
    {
      constexpr allocator () noexcept {}

      constexpr T *allocate (size_t n)
      { return static_cast<T *> (::operator new (n * sizeof(T))); }

      constexpr void
      deallocate (T *p, size_t n)
      { ::operator delete (p); }
    };

    template <typename T, typename U = T &&>
    U __declval (int);
    template <typename T>
    T __declval (long);
    template <typename T>
    auto declval () noexcept -> decltype (__declval<T> (0));

    template <typename T>
    struct remove_reference
    { typedef T type; };
    template <typename T>
    struct remove_reference<T &>
    { typedef T type; };
    template <typename T>
    struct remove_reference<T &&>
    { typedef T type; };

    template <typename T>
    constexpr T &&
    forward (typename std::remove_reference<T>::type &t) noexcept
    { return static_cast<T&&> (t); }

    template<typename T>
    constexpr T &&
    forward (typename std::remove_reference<T>::type &&t) noexcept
    { return static_cast<T&&> (t); }

    template <typename T, typename... A>
    constexpr auto
    construct_at (T *l, A &&... a)
    noexcept (noexcept (::new ((void *) 0) T (std::declval<A> ()...)))
    -> decltype (::new ((void *) 0) T (std::declval<A> ()...))
    { return ::new ((void *) l) T (std::forward<A> (a)...); }

    template <typename T>
    constexpr inline void
    destroy_at (T *l)
    { l->~T (); }
  }
}

inline void *operator new (std::size_t, void *p) noexcept
{ return p; }

constexpr bool
foo ()
{
  std::allocator<int> a;
  auto p = a.allocate (2);
  std::construct_at (p, 1);
  std::construct_at (p + 1, 2);
  if (p[0] != 1 || p[1] != 2)
    throw 1;
  std::destroy_at (p);
  std::destroy_at (p + 1);
  a.deallocate (p, 2);
  return true;
}

static_assert (foo ());
